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Lieberman, whose vote is critical to the bill's prospects, expressed his opposition twice during the day: first in an interview with CBS, and more strongly later, according to Democratic officials, in a private meeting with Senate Majority Leader Harry Reid.

Reid, who is hoping to pass the legislation by Christmas, needs 60 votes to overcome Republican objections, and has been counting on Lieberman to provide one.

But appearing on CBS, Lieberman said of the Medicare proposal, "Though I don't know exactly what's in it, from what I hear, I certainly would have a hard time voting for it because it has some of the same infirmities that the public option did.

"It will add taxpayer costs. It will add to the deficit. It's unnecessary," he added of a provision that Reid last week hailed as part of a breakthrough between liberals and moderates.

Possible filibusterDemocratic aides, speaking on condition of anonymity, said Lieberman later told Reid he would support a Republican-led filibuster against the bill if it contained the Medicare provision or permitted the government to sell insurance in competition with private companies.

The same aides added that Lieberman had responded differently last week when Reid asked him privately about the proposed Medicare provision. "He voiced support for the idea," said one official. Lieberman's public comments last week were also generally favorable. The aides spoke on condition of anonymity, saying they was not authorized to comment publicly.

In an interview that aired Sunday night — but was recorded last week — President Barack Obama voiced optimism about his call for sweeping legislation to expand coverage while cracking down on the insurance industry.

"I think it's going to pass out of the Senate before Christmas," he told CBS' "60 Minutes."

If Lieberman follows through on his plans, he leaves Democrats with few options as they try and pass legislation before Christmas. Most obviously, they could strip out the provisions he opposes, and hope liberals overcome their inevitable unhappiness and vote for the bill. For that approach to have any chance of success, Obama would have to lobby heavily in its favor.

Quick compromise unlikely
The bill's supporters could turn to Republicans instead in search of support, but that is unlikely to produce a compromise in the next few days.

Democrats are not without political leverage, however. Lieberman lost the Democratic nomination for re-election the last time he ran, in 2006, then won a new term as an independent. Even so, he retains his chairmanship of the Homeland Security and Government Affairs Committee, a post he holds at the pleasure of the Democratic-controlled Senate.

On CBS, Lieberman pleaded with Democrats to start subtracting expensive proposals from the overhaul, saying, "We don't need to keep adding onto the back of this horse or we're going to break the horse's back and get nothing done."

While Lieberman drew most of the attention for his comments, Democratic Sens. Claire McCaskill of Missouri and Ben Nelson of Nebraska also expressed concern about the legislation.

"I'm concerned that it's the forerunner of single-payer — the ultimate single-payer plan, maybe even more directly than the public option," Nelson said of the Medicare proposal. By single-payer, he meant national health insurance run by Washington. Unlike Lieberman, Nelson participated in negotiations last week between liberals and moderates that produced the general framework that included the Medicare provision.

Nelson also is seeking stricter abortion restrictions than are currently in the bill.

"The whole reason we're doing this bill is to bring down cost, first for the American people in health care, and secondly for the deficit," said Democratic Sen. Claire McCaskill of Missouri. "So until we get the numbers back from the Congressional Budget Office, we're all on hold."

Asked if she would vote against the bill if it raised health care costs overall, she said, "Absolutely."

In the meantime, only a few moderates have come out against the Medicare plan. But in a legislative struggle that is a game of inches, Democrats need all 60 votes in their caucus, and they don't yet have them.

Senate GOP leader Mitch McConnell gave no indication of backing down. "With the American people as overwhelmingly opposed to this bill as they are, for the Democrats to basically, arrogantly take the position that we're going to ignore public opinion and jam this through before Christmas, I think that's really a stretch," said McConnell, R-Ky.

The early Medicare buy-in was part of a compromise reached last week when Senate Democrats dropped the idea of setting up a federal health insurance plan to compete with private insurers. Lieberman was a firm opponent of the bill's original plan for a public insurance option.

Many Democrats who had favored that public option only grudgingly let it go, in return embracing the Medicare proposal as an appealing way to help people 55 to 64 — a group often vulnerable to losing employer-based health insurance when it's needed the most.

Under the compromise, private nonprofit plans overseen by the federal government would be offered in the marketplace.

Sen. Jay Rockefeller, D-W.Va., said he was working with Lieberman and others on controlling Medicare costs, and he voiced confidence fellow Democrats could get past their divisions. Party leaders are pushing hard to finish the Senate overhaul legislation before Christmas and to begin negotiations with the House, which has passed its plan.